When a XEP-0372 mention is received, the linked pubsub item is looked after in cache, and
if found, it is send to mentioned entity with `mention` tag added.
However, this doesn't work in some cases (see incoming doc for details). To work around
that, `@user@server.tld` type mention are also scanned in body, and mentions are added
when found (this can be disabled with `auto_mentions` setting).
Mention are only scanned in "public" messages, i.e. for pubsub items, and not direct
messages.
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